A further practical study of the mean free path in a gas may be made with the bromine diffusion experiment.
In this experiment bromine vapour is allowed to diffuse through air at atmospheric pressure in a closed tube.
The average distance d that the bromine diffuses in a certain time t is found by measuring the progress of the ‘middle brown’ colour of the bromine gas.
In the left hand tube the bromine diffuses into an air filled tube when the tap is opened while in the right hand tube there is initially a vacuum.
There are no air molecules for the bromine to collide with and so the bromine molecules fill the tube almost instantaneously.
DANGER - THIS EXPERIMENT SHOULD NOT BE PERFORMED BY SCHOOLS